1. Field of the Invention
The invention concerns a process for synthesizing of polyethylene terephthalate (PET) from polyethylene terephthalate waste by cleavage with ethylene glycol, separation of bis(hydroxyethylene) terephthalate (BHET) from the cleavage mixture, and esterification and polycondensation of BHET with terephthalic acid. The invention also concerns a process for producing polyethylene naphthalate (PEN) from polyethylene naphthalate waste by cleavage with ethylene glycol, separation of bis(hydroxyethylene) naphthalate (BHEN) from the cleavage mixture, and esterification and polycondensation of the BHEN with naphthalene dicarboxylic acid.
2. Summary of the Related Art
PET is produced on a large scale in order to manufacture from it products such as textile fibers and packaging materials. After use of these products, considerable quantities of wastes consisting essentially of PET are generated. These wastes often include mixtures with other polymers such as polyvinyl chloride, polyolefins, and other polymers. For ecological as well as economic reasons, it is desirable to recycle the main component of the waste, namely PET, in a production process to permit unlimited use of the newly synthesized PET.
It is known that PET wastes can be cleaved with the help of ethylene glycol, and then the cleavage mixture can be subsequently filtered and subjected to polycondensation while distilling off ethylene glycol and reaction water (British Patent No. 610,136, U.S. Pat. No. 3,222,299, European Patent no. A 174,062). However, these methods are suitable only for pure, unused PET wastes without any additives. Otherwise, contaminated glycolyzation product or PET is obtained that is not suitable for most applications. The delustering agents present in fiber wastes can be removed by treating the cleavage mixture with active carbon, celite, bentonite, etc. and then filtering (Italian Patent No. 702,065). Most contaminants, however, require that the BHET formed by cleavage be subjected to recrystallization from water before renewed polymerization (British Patent No. 610,136, East German Patent No. 13,537, Japanese Patent No. B 70-41,215, Indian Patent No. 143,323, European Patent No. A 723,951) or that the BHET first be hydrolyzed to terephthalic acid and ethylene glycol, which are then purified by crystallization and distillation (European Patent No. A 641,366).
One problem is the separation of BHET from the cleavage mixture prior to the purification step. BHET is obtained as an amorphous to very finely crystalline mass that is difficult to separate. On a very small scale, starting with 1 to 2.5 g PET, BHET yields of 55% to 70% are obtained when using ethylene glycol and approximately 82% when using mother liquor by cooling to 0.degree. C. (German Patent No. B 1,052,394). When cooled rapidly to 90.degree.-100.degree. C. and then cooled slowly to 20.degree. C., yields of approximately 75% are obtained when starting from 20 g PET (Indian Patent No. 143,323). However, these results obtained on a small scale cannot be scaled up to an industrial level--a pasty mixture is obtained, and the liquid containing glycol cannot be separated completely from the paste.
In addition, it is also known that BHET synthesized from the monomer can be reacted with terephthalic acid in the presence of a catalyst to yield PET (British Patent No. 775,030, U.S. Pat. No. 3,070,575). Use of BHET obtained from PET waste is not mentioned.
It is also known that dimethyl naphthalene-2,6-dicarboxylate can be produced by cleavage of PEN with methanol or with ethylene glycol and subsequent ester exchange with methanol (U.S. Pat. Nos. 4,876,378 and 5,430,174). However, production of BHEN is not disclosed.
The object of the present invention is to improve the profitability of the production of high-quality PET from waste PET by cleavage with ethylene glycol and subsequent polycondensation on an industrial scale. In particular, the object of this invention is to provide conditions for the crystallization of BHET in the cleavage mixture so that only large crystals that can be separated almost completely from the liquid phase are formed. The object of this invention is also to produce PEN from waste PEN by cleavage with ethylene glycol and subsequent polycondensation.